The present invention relates to plasma jet apparatus and, more particularly, to plasma jet apparatus effective for cleaning contaminants from a conductive substrate.
Plasma jet devices have been used for applying coatings to conductive substrates. In such plasma jet devices, an arc is sustained across a stream of inert gas such as, for example, a mixture of helium and argon. The arc ionizes and heats the inert gas to produce a flow of ionized gas or plasma which is typically directed through an aperture toward a conductive workpiece. U.S. Pat. No. 3,179,783 discloses such a plasma jet device in which a powder to be coated on the surface of a workpiece is injected into the plasma jet. The workpiece is maintained at a positive potential with respect to the forward electrode of the plasma jet device to form a transfer arc therebetween which urges the ionized plasma jet, and the entrained powder, to impact the substrate at high velocity. The substrate is heated by the impinging hot plasma aided by the transfer arc. The powder is heated and melted by its residence in the hot plasma and the molten particles are impacted at high velocity onto the heated subtrate to form a coating thereon.
This patent also discloses the application of high voltage pulses to the workpiece in order to periodically create an energetic spark discharge for hardening the surface of the substrate which, in the example disclosed, is a mild steel capable of being work hardened by such treatment. There is also disclosure in this patent of using reversed polarities on either of the two power supplies therein and also of using AC or compounded AC and DC without any disclosure of the parameters or effects thereof.
U.S. Pat. No. 4,162,389 discloses the use of a cathode target for improving the smoothness and reducing the penetration of a weld bead. This patent makes no mention of cleaning.
U.S. Pat. No. 4,328,257 to Muehlberger et al, the disclosure of which is herein incorporated by reference, discloses a system and method for plasma coating a protective material on a superalloy substrate such as used, for example, in blades or buckets of gas turbines. The plasma coating operation, in which the applied voltages produce an anodic workpiece, is preceded by a cleaning operation in which the applied voltages produce a cathodic workpiece. During the cleaning operation, cathode spots at the ends of small arcs travel across the surface of the workpiece. The motion of the cathode spots is produced by the electric and magnetic fields generated by the arcs themselves aided by complex motion of the plasma gun and the workpiece to preferentially remove contaminants from the surface of such alloys.
Although we do not intend to be limited by a particular theory of operation of our apparatus and method, we believe that surface cleaning with a cathodic workpiece relies on the fact that the surface contaminants on superalloys such as, for example, on International Nickel alloy IN738, and particularly the oxides of such superalloys, are thin insulating layers having substantial dielectric constants. We believe that, when an electric field of sufficient magnitude is applied across such thin insulating layers, a dielectric stress is produced in the surface contaminants which is sufficient to initiate a discharge current by field emission. A large number of arcs terminating in cathode spots on the negatively charged workpiece are observed. In therory, at least, motion of the cathode spots self-induced by the electric and magnetic fields, aided by the relative motion of plasma gun and workpiece, should be sufficient to keep the arcs and their associated cathode spots moving at high enough speed over the surface of the workpiece that, although the contaminant layer is removed and the underlying metallic substrate of the workpiece is cleaned, the motion is rapid enough and continuous enough to prevent overheating and consequent damage to the subtrate.
We have observed that the arcs used for cleaning with the Muehlberger et al apparatus and process occasionally stick, or remain in one location, an excessive amount of time and thereby produce localized melting and overheating of the substrate. This can lead to pitting and loss of material as well as damagingly high temperatures, particularly in the vicinity of thin sections such as a bucket trailing edge or tip. If arc damage is suspected during the cleaning process, the workpiece must be manually examined for excessive loss of material and/or cracking. If such damage has occurred, there may be no alternative to scrapping of the bucket. Since buckets of superalloy metals are expensive objects, and since the cleaning operation takes place almost at the end of the manufacturing cycle when a bucket represents a substantial investment in labor as well as material, means for reducing or eliminating the scrap rate at this late stage in the manufacturing process is welcome.